Hour Meters growing apart

[Lift12]

My hour meters (built into the tachs) are inexplicably growing apart. 340 Sundancer, factory "Sea Ray" tachs, no other info. Fair to guess they at Teleflex?
Engines are 1998 (Boat is '99) Mercruiser 7.4 MPI, Bravo 1. This is my second season with it. Bought it with 22.2 hrs apart. End of last season 28.6 apart after 100 hrs use. This season, with only ~50 hrs use they are now 39.3 apart. I start them one after the other pretty much immediately and not always in the same order and always run both.

Other than the obvious possibility that one gauge is malfunctioning - What gives? Where is the signal sent from?

Thanks.

 

[jimmbo]

Are any Accessories tied to the Key Switch, where it has to be "on" for them to work?

 

[Lift12]

No, I wish it were that easy to explain Jim.
I haven't tried it on this boat but I believe the alarm for zero oil pressure would be on constantly if I left the switch in Run without the engine running. That's what happens on my other boat with 5.7/Alpha setup.

 

[Scott Danforth]

they are just like the clocks in your house. after a month, they all read differently. if it really bothers you, keep the key on on the slow hour meter.

 

[dubs283]

Very few times I've seen ecm hours match/be close to boat hr meters

If you perform proper routine maintenance on your engines/running gear all will last thousands of hours

Know a fella interested in a 98 330ec with 454s. 1500 hours on the tach meters, omg!

Breaks down to about 60 hrs a season

Maintenance, my guy. Maintenance

 

[muc]

Dash tachs aren't very accurate, if they can achieve 5% they are considered good.
The ones you have seem to be getting worse. You will need to live with it or replace them.
Accurate engine hours can be accessed with a scan tool connected to the ECM. As long as the ECMs have never been replaced.
To log hours on your tachs, they need 12 volt positive a 12 volt ground and a signal from (I'm guessing here because you didn't provide a engine serial number and I don't memorize what years had what ECMs --- but I'm pretty sure) the ignition coil. Because of this just turning the key on the low hours tach won't do anything.